If you're talking about a bolt-on style neck, I'm not sure if anyone makes a THREE piece. USA Custom Guitars makes a 2-piece, with graphite reinforcement strips in the neck parallel to the truss rod. Likewise, Warmoth reinforces their necks (see above) with steel bars, but I don't remember if they only do one-piece necks of if you can get a two-piece. You could always contact a custom luthier (DP, Kinal, Ken Benensee, Chris Stambaugh, FBB, etc.) to see if they would be willing to clamp a three-piece neck together for you with graphite for reinforcement.